In X-linked scurfy (sf') disease, male mice develop severe autoimmune lymphoproliferative disease that is fatal within twenty-one days of age The pathology of this disease results from chronically activated CD4 v T cells that over-produce a wide variety of cytokines However, the mechanism by which T cells are initially activated is yet unknown FoxP3, the gene mutated in scurfy disease, has been cloned and is a member of the forkhead/ winged helix domain family When wild-type FoxP3/scurfin is over-expressed in otherwise normal mice, the mice are immunocompromised The human homolog of FoxP3 has been cloned and is mutated in IPEX syndrome Males afflicted with IPEX develop multiple autoimmune diseases including IDDM and do not survive beyond a year of age Taken together, these findings are consistent with scurfin having a role as a regulatory protein in T cell activation One model suggests that the level of scurfin expression in T cells determines the threshold of TCR stimulation required for T cell activation The absence of scurfin generates T cells that are hyper-responsive to TCR stimulation and lowers the threshold of tolerance to self-Ags Over-expression of scurfin increases the threshold of TCR sensitivity such that T cells are non-responsive to TCR stimulation The experiments in this proposal are designed to test this model In Specific Aim 1, we will determine the effect of the level and timing of scurfin expression on the development of tolerance or disease in A) sfmutant/GAD65 TCR Tg mice or B) FoxP3 Tg/NOD mice In Specific Aim 2, we will determine the regions in the FoxP3 gene plomoter that are responsible for regulating scurfin expression during the transition of T cells from naive to effector cells This work will provide valuable insight into the regulatory pathway(s) that fail allowing for the development of autoimmune disease.